כל הנשמה תהלל י -ה (סוף ספר תהילים)
This is the final chapter of Tehillim that is said in the morning service. Traditionally, the understanding is that one is able to praise G-d through the increased appreciation of the heart to the refined aspects of the world (as evidenced by the various types of musical sounds mentioned in this chapter which are some of the refined aspects of the world).
However, Chaza”l (Eruvin 18b) provide a different perspective. They say that from the time of the destruction of the Bais HaMikdash it is enough for the world to only use the two-letter name of HaShem.
Thus, everyone (כל נשמה) praises using the two-letter name of HaShem. However, the four-letter name of HaShem is restricted to use by the Cohanim in the Priestly blessing as mentioned in the verse in Parshas Naso ושמו את שמי על בני ישראל (My special Name of four letters).
This is difficult to understand since what is the connection to the destruction of the Bais HaMikdash. The different groups of Cohanim and Yisrael also related differently to the four-letter name prior to the destruction of the Bais HaMikdash.
Tosfos in the name of Rabeinu Chananel explains that when the Gemara cites this verse from Tehillim, that the word נשמה is vocalized with a dagesh. Tosfos does not explain this, and Rabbi Epstein did not find anyone else that explained this difficulty.
Rabbi Epstein offers a possible explanation. The word נשמה has two possible etymological roots. One is the letters נשם, which implies a living spirit. And thus, in the verse of Tehillim it would imply a connection to one’s soul.
The second root is from שמם. In this verse the word reflects the Nifal construct (passive), implying a sense of destruction. According to this form, the intention of the verse in Tehillim would reflect that when the entire world will be in a state of destruction (כל הנשמה) after the future destruction of the Bais HaMikdash, then praise will be done exclusively using the two-letter name of HaShem.
Thus, Tosfos emphasized that the letter Mem has a dagesh, to understand that the word should be understood as having a root with two Mem’s from the root שמם, destruction (and not without a dagesh which would imply the meaning of נשם, soul).
